1. Field of the Invention
The present invention relates to a semiconductor accelerometer provided with stoppers to be batch-fabricated.
2. Description of the Background Art
One example of a conventional semiconductor accelerometer is described in an article entitled "A Batch-Fabricated Silicon Accelerometer", I.E.E.E. Transactions on Electron Devices, Vol. ED-26, No. 12, December 1979, pp. 1911-1917, as shown in FIG. 1.
In FIG. 1, this conventional semiconductor accelerometer comprises a silicon substrate 1, a rectangular silicon center mass or weight 3 arranged in the central portion thereof, a silicon cantilevered beam 2 for supporting the silicon weight 3 to the silicon substrate 1 with a gap 4 between the silicon substrate 1 and the silicon cantilevered beam 2 and the silicon weight 3, and a piezoresistor 5 attached to the surface portion of the silicon cantilevered beam 2.
When the accelerometer is subjected to an acceleration, the weight 3 is moved to deflect the cantilevered beam 2, thereby causing stress in the cantilevered beam 2. As a result, the piezoresistor 5 formed on the surface of the cantilevered beam 2 changes its resistance due to a piezoresistive effect. By detecting the change in the resistance, the acceleration can be detected.
In FIG. 2, there is shown a mounting structure of the above described sensor chip, i.e., the semiconductor accelerometer, including upper and lower stoppers 6 and 7 mounted to the upper and lower surfaces of the semiconductor accelerometer, for preventing the cantilevered beam 2 from breaking due to excessive acceleration applied to the beam 2 when, for example, dropping the accelerometer.
This conventional semiconductor accelerometer and its mount structure have several problems.
Firstly, in the manufacturing process of the accelerometer, no protector for stopping the excessive displacement of the weight is provided after the formation of the cantilevered beam before the formation of the stoppers. Therefore, the accelerometer must be handled carefully, so as not to break the cantilevered beam, and productivity is lowered.
Secondly, the stopper forming process is complicated, which increases costs. One of reasons why the accelerometer is formed from the semiconductor is to effect cost reduction per chip by fabricating many chips using a batch process, i.e., many chips are formed on a wafer and processed in the same time to obtain chips of stable quality and low cost. However, since the stoppers are attached to the upper and lower surfaces of the accelerometer after the formation of the cantilevered beam in the wafer process, as shown in FIG. 2, this advantage of batch fabrication is lost, and costs increase greatly.
Thirdly, it is difficult to form the stoppers accurately. To meet the design requirements of the cantilevered beam, the gaps between the stoppers and the weight are controlled, to be accurately formed as small as several .mu.m to several tens .mu.m in the structure shown in FIG. 2. The stopper are required to be formed accurately and attached to the accelerometer correctly. Hence, high technologies in preparing and bonding the stoppers are required and, thus, the cost increases.
Further, in the accelerometer described above, an additional metal weight may be attached onto the silicon weight 3 in order to minimize the sensitivity of another axis, but the thickness of this additional metal weight tends to vary and to deteriorate the accuracy of the gap between the stoppers and the metal weight on the silicon weight.
FIG. 3 illustrates one example of a conventional method of manufacturing semiconductor accelerometers such as the one described above using an electrochemical etching technique, as disclosed in Japanese Patent Laid-Open Specification No. 61-97572, entitled "Method of Manufacturing Semiconductor Accelerometers". In this method, an n-type silicon is used as an anode, and a p-type silicon is selectively etched using an alkali etching solution.
In FIG. 3(a), first, an n-type silicon layer 12 and a p-type silicon window 13 of predetermined thicknesses are formed on a (100) surface of a p-type silicon substrate 11. The n-type silicon layer 12 is formed by using a thermal diffusion method or an epitaxial growing method, while the p-type silicon window 13 is formed by using the thermal diffusion method or by leaving a part of the p-type silicon layer when the n-type silicon layer 12 is formed in the p-type silicon layer by the thermal diffusion method. A piezoresistor 5 is formed in a predetermined surface region of the n-type silicon layer 12 by using the thermal diffusion method or ion implantation method.
In FIG. 3(b), a SiO.sub.2 or Si.sub.3 N.sub.4 film acting as a silicon etching mask is formed in predetermined regions on the back of the p-type silicon substrate 11. Then, while the n-type silicon layer 12 is used as an anode, the selective etching of the p-type silicon substrate 11 is carried out in the electrochemical etching method using the alkali etching solution. As a result, the silicon support 1, the silicon weight 3, the silicon cantilevered beam 2 and the gap 4 are simultaneously formed.
This conventional method of manufacturing semiconductor accelerometers has the following problems:
(1) The electrochemical etching method exhibits excessive anisotropy, and the etching is stopped by a (111) surface. Therefore, as shown in FIG. 4, the stress may concentrates in corners Y and break the silicon cantilevered beam 2 due to the concentrated stress.
(2) The etching is stopped by the (111) surface, and a p-type silicon structure such as the one shown in FIG. 5 may not be etched, thus limiting the structure to be processed.